Due to the fact that flue gas emitted from a boiler, such as one that is a combustor in a thermal power plant, for example, contains highly toxic mercury, various systems for reducing mercury contained in the flue gas have been developed.
A boiler typically includes a wet-type desulfurizer for reducing sulfur contained in the flue gas. It is widely known that, in a flue gas control facility including such a boiler having the desulfurizer provided thereto as an air pollution control apparatus, if the amount of chlorine (Cl) contained in the flue gas increases, the ratio of divalent metallic mercury that is water soluble increases, and the desulfurizer can collect mercury more easily.
Various inventions related to a method or an apparatus for controlling metallic mercury by combining a NOx removing apparatus that removes NOx and the wet-type desulfurizer that uses an alkali absorbent as a SOx absorbent have been devised recently (Patent Document 1).
A reducing method using an adsorbent such as activated carbon or a selenium filter is commonly known as a method for controlling metallic mercury contained in flue gas. However, because such a method requires a special adsorbing-reducing unit, the method is not suited for controlling a large volume of flue gas from a power plant, for example.
As a method for controlling metallic mercury contained in a large volume of flue gas, a limestone-gypsum method using a gas-liquid contact type desulfurizer has been widely employed as a method for reducing SOx through reactions expressed by formulas (1) and (2) below:SO2+CaCO3+1/2H2O→CaSO3.1/2H2O+CO2 (absorption)  (1)CaSO3.1/2H2O+3/2H2O+1/2O2→CaSO4.2H2O (oxidization)  (2)
[Patent Document 1] Japanese Patent Laid-open No. 2007-7612